The present invention relates generally to an apparatus and method for using electromagnetic energy as the means of automatic data collection (ADC). More particularly, the invention relates to an apparatus and method for using frequency modulated (xe2x80x9cFMxe2x80x9d) sideband electromagnetic energy to enable full duplex communication with another communication device.
Radio frequency (xe2x80x9cRFxe2x80x9d) remains the sole medium for conducting wireless communication. Despite the efforts of those in the fiber-optics industry to portray light as a wireless medium, it is still a materially bound, point-to-point connection that requires the use of cable.
In the RF domain, communication devices are further subdivided by their power source. The subdivision is generally based upon whether the communication device is xe2x80x9cactivexe2x80x9d or xe2x80x9cpassive.xe2x80x9d An active communication device requires an external power source such as direct current (xe2x80x9cDCxe2x80x9d) from a battery or alternating current (xe2x80x9cACxe2x80x9d) from a power grid, for example. In contrast, a passive device draws its power from ambient energy such as solar, wind, or ambient RF, for example.
Active devices have the most versatility due to their general lack of power constraints. However, in many applications an external power source is not practical. For example, in a vehicle navigation application where transponders are placed along a roadside, an AC connection to each transponder is cost prohibitive and a DC source causes severe maintenance problems.
Some of the many commercially active areas that are currently using passive wireless communications are Radio Frequency Identification (xe2x80x9cRFIDxe2x80x9d), microcellular, and networking, for example.
RFID uses radio frequency transmission to identify, categorize, locate, and/or track people, animals, and objects. An RFID system is commonly composed of three components: an interrogator (reader), a transponder (tag), and a data processing system such as a computer.
The simplest form of RFID products can be compared to an electronic bar code. They operate by the interrogator transmitting a RF wave to the transponder. The transponder then absorbs the RF energy from the interrogator to change an internal power tank circuit. Sufficient energy is stored, the transponder transmits its stored code on a known frequency to be received by the interrogator. The data processing system then interprets the code.
More sophisticated RFID products interface with external sensors for measuring various parameters, including Global Positioning Systems (xe2x80x9cGPSxe2x80x9d) which track objects. In these systems, the code transmitted by the transponder is variable.
One limitation to currently available RFID systems is that the interrogator and transponder communicate in an atomic sequence, i.e., one transponder at a time. The interrogator""s RF field must be constrained to the general area expected to be occupied by the desired transponder. If two transponders are present, the read operation will fail.
In addition current RFID systems have the limitation that only one interrogator may acquire a transponder at any time. RF fields from closely adjacent interrogators will interfere with each others operation.
Further, even low levels of RF energy are very leaky and propagate into unexpected areas by unexpected means, commonly referred to as electromagnetic interference (xe2x80x9cEMIxe2x80x9d). Since the radio frequency spectrum is shared and busy, the interrogator/transponder communication channel is susceptible to disruption from random sources of RF. Therefore, current RFID technology relies on close proximity of the interrogator/transponder pair and exceedingly low power levels of operation to limit EMI. This has limited the art to a range of approximately one and one half meters maximum.
Sophisticated systems generally require an active system to support the required peripherals. Further, when used for tracking and locating, RFID remains expensive due to the extensive support peripherals required, such as GPS.
In microcellular applications, the current direction being explored is to use the harmonic generation and opto-electronic mixing properties of Mach-Zenhnder modulators to generate modulated subcarrier signals at high-order harmonics of the input signals. This permits the simultaneous transmission over optical fiber of a modulated and an unmodulated signal, both at high-order harmonic frequencies of the input signals, for the purpose of transmitting both a local oscillator tone and the modulated signal required at a base station for microcellular applications, see IEEE Transactions on Microwave Theory and Techniques, March 1996 v44 n3 p446(8).
Though this technique solves the problem of simultaneous communication of the modulated and unmodulated signals, it does so by constraining the transmission medium. Thus, this technique can not be applied to RF and even assuming that it could, it would require active devices on both ends.
In one commercially available system provided by RadioLAN, Inc., a RF networking card is used in a laptop computer to link the laptop to a local-area network (xe2x80x9cLANxe2x80x9d). The transmission rate can be at 10 Mbps Ethernet speeds, but its use of proprietary protocols and non-standard narrow-band frequency makes it useless in multi-vendor installations.
One approach that has been taken in the wireless industry has been the use of single-sideband transmission. In these systems, single-sideband is simply a sophisticated form of amplitude modulation. When RF and audio frequency (xe2x80x9cAFxe2x80x9d) signals are combined in a standard amplitude modulation (xe2x80x9cAMxe2x80x9d) transmitter (such as one used for commercial broadcasting) four output signals are generated: the original RF signal, called the carrier; the original AF signal; and two sidebands, whose frequencies are the sum and difference of the original RF and AF signals, and whose amplitudes are proportional to that of the original AF signal.
The sum component is called the upper sideband. The upper sideband is erect, in that increasing the frequency of the modulating audio signal causes a corresponding increase in the frequency of the RF output signal.
The difference component is called the lower sideband, and is inverted, meaning an increase in the modulating frequency results in a decrease in the output frequency.
All of the intelligence is contained in the sidebands, but two-thirds of the RF power is in the carrier. The carrier serves only to demodulate the signal in the receiver. If this carrier is suppressed in the transmitter and reinserted in the proper phase in the receiver, several significant communications advantages accrue. If the reinserted carrier is strong compared to the incoming double-sideband signal, exalted carrier reception is achieved in which distortion caused by frequency-selective fading is greatly reduced. Also, the lack of a transmitted carrier eliminates the heterodyne interference common to adjacent AM signals. Perhaps the most important advantage of eliminating the carrier is that the overall efficiency of the transmitter is increased. The power consumed by the carrier can be put to better use in the sidebands.
In normal AM transmission, the power in the carrier is continuous and an AM transmitter requires a heavy-duty power supply. A double sideband transmitter having the same power output as an AM transmitter can use a much lighter power supply because the duty cycle is low. A single-sideband transmitter can, therefore, achieve the same effective range using one third of the output power as a standard AM transmitter.
A problem with AM is that it gives bursts of RF that are not always clear. Further, it is not as easy to detect shifts in an AM signal. This is particularly true with moving targets because amplitudes decrease proportionally to the distance to the transmitter.
Accordingly, it is an object of this invention to provide a wireless communication system that is adaptable to multiple applications.
It is another object of this invention to provide a wireless communication system that is power limited for passive applications.
It is still another object of this invention to provide a wireless communication system that is full duplex and can include unmodulated signals.
It is a further object of this invention to provide a wireless communication system that works in the presence of EMI, multiple transponders and/or interrogators.
It is still another object of the invention to use single-sideband transmission to increase range in passive RF systems but to do so without significantly decreasing the signal-to-noise ratio.
These and other objects of the invention will be obvious and will appear hereinafter.
The aforementioned and other objects are achieved by the invention which provides a communication system and a method associated therewith. The communication system comprises a transmitter, a transponder, and a receiver.
The communication system described herein blends techniques in use by cellular and networked systems architectures into one. The protocol allows multiple RF targets and interrogators to coexist and function in a shared airspace through error handling and backoff algorithms similar to Ethernet. Thus completing a system for duplex data communications and simplified radio architecture.
The transmitter is adapted to transmit on a predetermined frequency a first sideband and adapted to have first frequency-modulated data encoded thereon.
The transponder then receives the first sideband and transmits on the predetermined frequency a second sideband disposed in a band distinct from the first sideband. The transponder is also adapted to have second frequency-modulated data encoded thereon.
The receiver then receives the second sideband on the predetermined frequency. The second frequency-modulated data is then decodable from the signal to complete the bi-directional communication.
In addition, the concentration of most of the RF power into natural sidebands and the reliability of FM propagation equates into greater operational range and design flexibility.
In further aspects, the invention provides methods in accord with the apparatus described above. The aforementioned and other aspects of the invention are evident in the drawings and in the description that follows.